A device for mixing liquids and flowable solids while they tranverse a pipe section of, for example, circular cross section, can comprise a mixing element received within the pipe section and subdividing the flow of material into at least two streams while guiding them around and along a common axis.
Such a device can be formed from a flat sheet-metal blank which can be provided with generally flat surface regions of triangular outline.
The term "mixing element" as used herein is intended to refer to a static structure across which the stream to be mixed is passed, such element generally being provided in a fixed condition within a pipe through which the material is displaced, e.g. by a pump or other means.
The term "flowable material" is used herein in its most general sense to mean any fluid or flowable solid, although it is particularly intended to refer to materials which are difficult to mix and are relatively viscous. The nature of materials which may be treated with the system of the present invention will be detailed below, but it should be understood that the treatment may involve any conventional treatment which utilizes the movement of such flowable materials.
Thus the term may refer to homogenization, material exchange, heat exchange or a combination thereof whereby, for example, a flowable solid may be treated by a liquid, a liquid may be treated with a gas, a solid can be treated with a gas, or various heat exchange and material exchange or chemical reaction processes can occur with or within the flowable materials.
Thus in the instant description, the liquids and flowable solids to be mixed can be subjected to a process in which each particle or portion of the liquid or of the solid in the medium comes into contact with a surface of the device which guides the flow and which induces a rotary movement therein. The particles are also brought into contact with other particles of the liquid or the solids.
The mixing process of the present invention may thus also involve a heat exchange or an interchange of matter of interaction between the particles themselves or between the particles and fixed walls of the device, or between particles of the flowable material or layers arranged on or formed as part of the mixing device. For example, when the interchange is a catalytically induced chemical reaction, portions of the device may be constituted as a catalyst support.
The "mixing" can thus include kneading, emulsifying, dispersing, plasticizing or homogenizing a flowable mass thereby retaining or altering physical or chemical properties. The production of a uniform molecular weight in a flowable synthetic resin of liquid or particulate form is thus a mixing process in the sense of the present invention.
Furthermore, if the reaction involves a catalyst on a wall or pipe surface a mixing process nevertheless takes place in order to bring all of the particles of the flowable material into as uniform contact as possible with the catalyst as the streams traverse the pipe section.
The mixing can occur during polymerization, condensation, neutralization or reduction, during oxidation or hydration, during fermentation or like processes.
Layers of an adsorption agent, a grinding or polishing agent, or any other material-treatment agent may be provided on the surfaces of the device. A case in point is the dehusking of grain in which the flowable mass of grain, with husks or hulls thereon, is cause to traverse a device of the present invention in a uniform flow so that the grains of corn or rice, etc. are brought into uniform contact over their entire surfaces with solid grinding or abrading surfaces within the pipe section to carry out the treatment.
Devices utilizing the principles described above are known from various applications and mention may be made, for example, of German No. 3,861, No. 86,622 and No. 1,557,118. In these systems, for the purpose of heat exchange or to mix flowable materials it is known to provide several successive and oppositely twisted mixing elements in the form of short helically bent strips into a pipe or duct to internally subdivide the flow of fluid into two flow cross sections of uniform area.
The adjacent end edges of the successive elements are arranged at an angle with one another to repeatedly subdivide the streams and combine them.
Each flow cross section or stream thus can contain parts of the divided flows from the preceding mixing element.
It is also known (see German open application, Offenlegungsschrift, Nos. 2,205,371 and 2,320,741) to mix elements in the form of layers in contact with one another to form a multitude of flow channels. In this case, the longitudinal axes of the individual flow channels within each layer are parallel, at least in groups. The longitudinal axes of the flow channels of adjacent layers can be inclined to one another. Between the individual layers, exchange may occur between the respective streams of the flowable material through openings.
German Pat. No. 2,058,071 and U.S. patent No. 3,804,376 describe systems for locking mixing elements in a pipe more firmly into position and provide a configuration which enables these elements to be manufactured more easily. In these systems twisted strip elements are provided and have a slit for engagement with adjacent or successive strip elements.
Mention should also be made of French Pat. No. 2,209,601 which provides a pipe section with bent sheet-metal mixing elements. In these mixing elements, triangular flat sections are provided and the triangular surfaces or zones are of different shape and size with all of the triangle vertices terminating at a common point. Fold lines are provided between these triangular sections.
Experience has shown that the mixing elements of this French patent do not bring about a uniform splitting and rotation of the flow material over a significant axial length, especially because the hydraulic diameter of the flow cross sections traversed by the streams into which the mixing element splits the flow are not constant over the length of the mixing element.
Disadvantages also have been found with systems of the type described in the German printed application (Auslegeschrift) 1,557,118 mentioned briefly above.
In all mixing processes, the shearing action of the respective mixing element has been found to determine the success or efficiency of mixing as well as the effectiveness of the subdivision of the incoming stream of flowable material into flow parts or streamlets.
According to the type of loading, a change of shape and position of the folded layers of material which slide on one another can be achieved. The type and intensity of the loading is dependent on the respective constructions of the flow channel which is formed by the mixing elements built into the portion of the pipe through which the flowable material passes.
In known devices in which the individual flow cross sections are of semicircular configurations and constitute the partial flow channels, an unchanging ratio between separating and shearing action is obtained. This ratio remains substantially constant even with the change in the pitch of the mixing element. In such systems, if it is desired to increase the shearing action to provide a certain degree of shearing within a particular material, i.e. to match the desired properties of the material to the mixing device, it is necessary to increase or otherwise alter the number of mixing elements.
In practice, therefore, the devices of the prior art must be provided with numerous mixing elements and relatively long mixing paths. This is especially the case when the element can have the length of 1.25 to 1.5 times its diameter, such a length having been found to be convenient from the point of view of manufacture.
Difficulties have also been encountered in deforming the elements to form helically curved strips. These difficulties increase significantly as a result of extreme transfers and longitudinal distortions of the strip with increasing diameter. There is, therefore, a dependence between the thickness of the material and the diameter of the elements which can be fabricated therefrom. In twisting a conventional steel such as the V to A steel the thickness of the element must be about 0.075 times the diameter in order to avoid tearing or undesired deformation of the element upon helical twisting. This has been found to rule out largely the manufacture of such elements in large diameters from strip material. In practice one finds that it is necessary in manufacturing large diameter mixing elements, to apply casting techniques which are far more expensive and complex.